Process for desulphurizing metal and metal alloy particles



3,256,088 PRGCESS FOR DESULPHURIZENG METAL AND METAL ALLQY PARTICLESVladimir Nicolaus Mackiw, Fort Saskatchewan, Alberta, David J. I. Evans,Edmonton, Alberta, and Vasyl Kunda, Fort Saskatchewan, Alberta, Canada,assignors to Sherritt Gordon Mines Limited, Toronto, Ontario, Canada, acorporation of Canada No Drawing. Filed Nov. 9, 1962, Ser. No. 236,631 5Claims. (Cl. 75224) This application is a continuation-in-part of anapplication Serial No. 82,166, filed January 12, 1961, now abandoned.

This invention relates to the desulphurization of finely dividedparticles of metals, metal alloys and composite metal coated metal andnon-metal compounds and mixtures and products thereof. It isparticularly directed to providing a new and useful process for removingsulphur from metals and metal alloys in powder, or finely divided form,or in the form of wrought products formed of compacted finely dividedparticles by reacting the product in solid state with hydrogen gas underconditions which establish and maintain a rapid and efficientdesulphurizing reaction without adversely altering desired physicalcharacteristics of the metal or metal alloy.

Methods are known for producing metals and metal alloys in powder orfinely divided particulate form. Conventional methods involve mechanicalattrition, such as grinding, spraying, sputtering and the like, ofconventionally produced metals and metal alloys. Also,hydrometallurgical methods are known by means of which product metals ofthe group silver to cadmium inclusive in the electrochemical series ofthe elements, and which are capable of forming a soluble ammine complex,can be precipitated from solutions in which they are present asdissolved salts by reacting the solutions with a sulphurfree reducinggas, such as hydrogen, at elevated temperature and pressure. This latterprocess possesses the important advantage that the metal or metal alloyis produced in a very finely divided form which has desired physicalcharacteristics for such uses in powder metallurgy as compacting, flamespraying and the like. Also, methods are now known for producingcomposite, metal coated metal and non-metal compounds.

The presence of sulphur as a contaminant in powder metals and composite,metal coated metal and non-metal compounds is often objectionable. If itis present in amount in excess of market specification set for aparticular product, it may be unsuitable for its intended use or atleast it may be subject to a reduction in its market value. The sulphurcontent of powder metals and metal alloys must be low.. For example, thesulphur content of powder nickel and copper should be below about 0.02%for normal commercial use and may be required to be below about 0.005%for special uses. Regardless of the origin of the powder metal or metalalloys, whether produced by conventional pyrometallurgical and/orelectrolytic processes, or by now known hydrometallurgic'al processes inwhich the metal powder is precipitated from a solution in which it ispresent as a dissolved salt by reaction with a reducing gas, it mayrequire treatment to reduce the sulphur content safely below a maximumestablished for special uses. A known method of desulphurizing metalsand metal alloys involves the step of treating a molten bath of themetal or metal alloy with lime, CaO, or limestone, CaO This procedurehas the important disadvantage that the desired physical properties of achemically precipitated product powder metal or metal alloy aredestroyed in melting.

We have found that the sulphur content of the metal or metal alloyparticles and compacted products formed therefrom can be rapidly loweredto below 0.02 weight percent by heating them, in particulate form, in arelatively porous adherent form, such as when compacted into porousbriquettes, or into green strips, or in a compacted form less than 0.25inch thick and of a density substantially of the theretical density suchas is obtained, for example by hot working green compacts into wroughtproducts, in a stream of hydrogen gas containing less than 0.15%hydrogen sulphide at a temperature below the melting temperature of themetal or metal alloy subjected to treatment.

The process of this invention is independent of the source or origin ofthe compacted or uncompacted particles subjected to treatment.- That is,they may have been produced by conventional pyrometallurgical and/ orelectrolytic processes followed by spraying, sputtering or mechanicalattrition to produce particles within a predetermined size range or theymay have been produced by reacting solutions in which they are presentas dissolved salts with reducing gas at elevated temperature andpressure.

The efficiency of the desulphurizing reaction is a function of the depthof the bed of particles or the thickness of the compacted form orwrought product, the temperature at which the reaction is conducted, andthe rate of flow of hydrogen.

It is found that the best results, having regard to the overall cost andtime of the treatment, are obtained when the reaction is conducted at atemperature above about 1000 F. For general use, therefore, the processis best suited for the treatment of metal or metal alloy products whichhave a melting point above 1000 F. Copper, silver, nickel, cobalt, iron,chromium and vanadium are illustrative of such metals. However, it willbe understood that the process can be employed to treat lower meltingpoint metal and metal alloy products, such as lead, if their marketvalue warrants the cost of treatment which results fromthe substantiallylonger reaction times required, such as in the case of high puritymetals the cost of production of which is only incidental to the valueof the product in its desired end use.

The surface areas of the products exposed to the desulphurizing gas isimportant. The maximum surface area of the product must be exposed tothe hydrogen gas. It is found that metal or metal alloy particles can betreated with advantage in unconsolidated form, such as by feedinghydrogen at atmospheric or above atmospheric pressure into a reactionzone which contains the particles, or in the form of relatively porousbriquettes or green shapes of the types formed in the initial stages ofcompacting processes. In such briquettes and green shapes, individualparticles are bonded to adjacent particles by contacting surfaces withspaces or voids between noncontacting surfaces through which thehydrogen gas can pass and circulate in contact with exposed surfaces ofthe particles. It has been found, also, that compacted products ofsubstantially 100% density and less than u about one-quarter inch inthickness can be treated with advantage by this process.

The steps involved in the production of useful products of substantially100% of the theoretical density from metal or metal coated particlesinvolve, usually, the formation of the particles, by an initialcompacting step, into a green shape of from about 45% to about 95% ofthe theoretical density. The green shape is sintered, usually at atemperature near but below the melting temperature of the particles. Thesintered shape is then hot or cold worked to form a useful product ofsubstantially 100% of the theoretical density of the particles. Thedesulphurizing reaction can be conducted prior to, during, or after anystep of the overall process. For example, it can be conducted on thestarting material in its finely divided form; prior to, during or afterthe sintering step; or prior to, during or after any of the hot or coldworking steps. In the case of wrought or partially wrought products ofsubstantially 100% of the theoretical density, the thickness preferablyshould be less than about 0.25 inch to ensure satisfactorydesulphurization by the process of the present invention. There areseveral inherent advantages in conducting the desulphurizing treatmentin the early stages of the overall process before the product has beenfully densified such as a fast reaction rate and the possibility ofcombining it with, for example, the sintering step or a heating step.

The desulphurizing reaction can be expressed by the following equation:

in which Me is a metal. The sulphur usually is present as combinedsulphur, such as a sulphide, as in the above equation, or as a sulphate.Metal sulphates or other sulphur bearing salts present initially in themetal are reduced to metal sulphides under the conditions ofdesulphurization. Thus, the above equation is equally applicable. Thereaction is reversible. Thus, it is necessary to establish and maintaina partial pressure of hydrogen in the reaction zone which issubstantially higher than the partial pressure of hydrogen sulphide.This necessitates supplying hydrogen in substantial excess of that whichis required for combination with the sulphur present in the metal todrive the reaction in the direction of hydrogen sulphide formation andto continuously remove the so-formed hydrogen sulphide from the reactionzone to maintain a substantially hydrogen sulphide-free atmospherethroughout the reaction zone. We have found, for example, that in thedesulphurization of nickel briquettes at 1500" F., the presence of 0.15%H S by volume in the hydrogen prevented the removal of sulphur.objective is attained by conducting the desulphurizing reaction in acirculating stream of hydrogen such that hydrogen is continuously fedinto and hydrogen sulphide containing hydrogen gas is continuouslywithdrawn from the reaction zone. The hydrogen sulphide can beseparated, if desired, from the gas withdrawn from the reaction zone byknown methods and the hydrogen sulphide-free gas returned to thereaction zone. The hydrogen can be supplied in relatively pure form orin dilute form, such as cracked ammonia which contains about 75%hydrogen by volume. It is supplied in substantial excess of thatrequired to supply the hydrogen required for combination with thesulphur to drive the reaction to form hydrogen sulphide. In the absenceof an excess of hydrogen, the reaction will cease, resulting in anundesired high sulphur content in the desired metal or metal alloyproduct.

The following examples illustrate the results which can be obtained inthe operation of this desulphurizing process. Where metal particles wereused, they were of a particle size smaller than about 300 microns.Elsewhere, the dimensions of the products treated are as stated. Allpercentages are by weight unless otherwise noted.

This

Example 1 To evaluate the sulphur reduction of porous nickel briquettesof compacted nickel metal powder of about 75 of their theoreticaldensity, a number of tests were conducted under varying conditions assummarized by the following examples:

256 grams of nickel briquettes, approximately 1 /2 long, 1%" wide and /8thick, of about 70% density and containing an average of 0.018% sulphurby weight were heated at 1600 F. in a stream of hydrogen flowing at therate of 0.075 standard cubic feet per minute per pound of nickel withvarying retention times to yield the final sulphur contents at thesurface and interior of the charge as indicated by Table I herebelow.

TABLE I Final Sulphur Content Sample N 0. Retention Time (Mina) InteriorSurface In this series of tests, the sulphur content in the interior ofthe briquette was reduced to 0.008% in 30 minutes. The sulphur contentat the surface of the briquette required between 30 to 45 minutes forsulphur reduction below 0.008%. This teaches that the desulphurizationprocess can be employed to reduce the sulphur content of porous butadherent compacted shapes.

Example 2 Six 50 gram nickel powder samples of a particle size smallerthan 300 microns and averaging 0.017% sulphur were heated for 60 minutesin a stream of hydrogen flowing at a rate of 0.035 standard cubic footper minute per pound of nickel at temperatures within the range of about2400 F. to 1300 F. to yield the following sulphur contents:

The efiiciency of desulphurization in a constant period of treatmenttime is a function of temperature and, as indicated by the resultstabulated in Table II, treatment for 60 minutes at a temperature ofabout 1300 F. provided a sulphur content below 0.008%

Example 3 Samples of compacted nickel powder in the form of green strips0.035 inch thick and of about of the theoretical density were sinteredin a stream of hydrogen in bundles of seven strips at temperaturesranging from 2000 F. to 1600 F. for from 5 to minutes. The stripsaveraged 0.025% sulphur by weight before desulphurization. Sintering wasconducted in a substantially hydrogen sulphide-free hydrogen gas byfeeding hydrogen into the sintering zone and exhausting hydrogensulphide bearing gas from the sintering zone.

Table.

III below indicates the final sulphur contents of the individual stripsand bundle averages:

powder and briquettes.

6 it is in the treatment of uncoiled strips, unconsolidated TABLE IIISulphur Content (Percent by Weight) Tempera- Retention Sample No. tureF.) Time Strip No.

(Mins.) Average Table III indicates that at least 15 to 3 0 minutes areExample 6 necessary for desulphurizing the compacted nickel strips tobelow 0.008% sulphur at temperatures ranging from 1600 F. to 2000 F.

Example 4 Cobalt metal strips formed by compacting cobalt powder intogreen strips 0.035 inch thick and of about 90% of the theoreticaldensity were heated in bundles of seven strips at 1800 F. for 30 and 60minutes in a stream of substantially hydrogen sulphide-free hydrogen.The average sulphur content before desulphurization was 0.035%.Desulphurization results are set out in the fol- Cobalt metal strips ofsubstantially 100% density, were lowing Table IV. prepared in a similarmanner and subjected to the same TABLE IV Sulphur Content (percent byWeight) Sample Retention No. Time Strip No.

(Mins.) Average 1 2 3 I 4 5 6 7 VI-l .005 .005 .005 .006 .005 .005 .0050. 0053 vI-2 .005 .005 .005 .005 .005 .005 .005 0.005

- Sulphur reduction to 0.005% was effected within 30 45 treatment. Thedesulphurization results for the cobalt minutes of heating at 1800 F.Subsequent treatmentby prolonged heating in the hydrogen gas did notmaterially reduce the sulphur content.

Example 5 Nickel powder of a particle size smaller than 100 microns wasroll compacted to form a green strip 4 /2 inches wide and 0.04 inchthick. The strip had a density, of about 90% of the theoretical andcontained about 0.02% sulphur. The green strip was wound on a spool asit emerged from the compacting rolls to form a coil. Coils of this stripwere heated in a closed furnace at 1650 F. for varying times in anatmosphere of hydrogen. Cracked ammonia, which contained 75% hydrogenand 25% nitrogen by volume, was admitted to the furnace at the rate of20 litres per minute. The results of this treatment are set out in TableV below.

TABLE V Sulphur Content (Percent) Time Weight. (Mins) lbs.

Interior Exterior This example illustrates that the desulphurizingtreatment is an effective in the treatment of coiled metal as metalstrips are also included in Table VI.

TABLE VI Average Sulphur and Carbon Contents (Percent by Weight)Retention Time (Mius) Nickel Strips Cobalt Strips S C S C Sulphurreduction to 0.005% was effected within 30 minutes of heating at 1500 F.for the nickel strips. Sulphur reduction to 0.004% was effected within60 minutes of heating at 1500 F. for the cobalt strips. This exampleillustrates that the desulphurizing treatment is as effective on wroughtproducts of a density substantially of the theoretical density as it ison unconsoli dated powder and porous, adherent, compacted powder such asbriquettes and green strip. It also illustrates under the conditions ofoperation hydrogen also combines with carbon present in the product andthat the carbon content of the product is lowered concurrently with thesulphur content.

We have found also that there is a definite minimum hydrogen flowvelocity above which optimum results are obtained in the desulphurizingreaction. We found that when the hydrogen flows through the reactionzone at a velocity below about 400 centimetres per minute, thedesulphurizing reaction is ineflicient in that the reaction rate is tooslow for operation on an economically practical basis. For example, at avelocity of about 352 centimetres per minute, 12.7% of the sulphur wasremoved in minutes from nickel briquettes having a density of about 70%of the theoretical density and which contained originally 0.22% sulphur,and only 48.0% was removed in two hours. By increasing the velocity ofthe hydrogen flow to 1760 centimetres per minute, 43% of the sulphur wasremoved in 30 minutes and 85% was removed in two hours. Thus, inaddition to increasing the rate of sulphur removal, a substantial savingof hydrogen is obtained by flowing the hydrogen through the reactionzone at a velocity greater than about 400 centimetres per minute.

The velocity at which hydrogen flows through the reaction zone above 400centimetres per minute is a matter of selection influenced by operatingeconomics. However, based on the increase in the rate and the efiiciencyof the reaction, we found that the use of hydrogen velocities above 3000centrimetres per minute is not necessary or warranted.

The present desulphurizing process possesses a number of importantadvantages. It can be inexpensively conducted. It can be employed toreduce the sulphur content of particulate solids in unconsolidated form,in the form of porous compacts or of products of thin section ofsubstantially 100% density. It can be conducted also simultaneously witha sintering operation in which porous compacts are sintered to improvetheir workability in subsequent treatments.

It will be understood, of course, that modifications can be made in theembodiments of the process described herein without departing from thescope of the invention as defined by the appended claims. The specificconditions of times and temperatures under which optimum sulphur removalis obtained can be readily determined for specific sizes and shapes ofspecific metal and metal alloy products from the teachings set outhereinabove.

We claim:

1. A method of rapidly and efliciently desulphurizing a materialselected from the group consisting of non-ferrous metallic particles ofa size smaller than about 300 microns and compacted products formedtherefrom and which contain at least 0.02 weight percent sulphur as acontaminant which comprises heating a layer of the sulphur contaminatedmaterial in a reaction zone at a temperature below the melting point ofsaid material but above 1000 F.; flowing hydrogen gas containing lessthan about 0.15% hydrogen sulphide by volume through said reaction zoneat a velocity greater than about 400 centimetres per minute; continuingsaid heating in said flowing hydrogen for a period of time up to about 2hours to lower the sulphur content of said material to substantiallyless than about 0.02 weight percent.

2. The process according to claim 1 in which the sulphur contaminatedmaterial consists of individual -un compacted non-ferrous metalparticles.

3. The process according to claim 1 in which the sulphur contaminatedmaterial consists of non-ferrous metal particles formed into a compacthaving a density within the range of about 45% to about 95% of thetheoretical density of the metal constituting the particles.

4. The process according to claim 1 in which the contaminated materialconsists of metallic particles formed' of a metal selected from thegroup consisting of silver, copper, nickel, cobalt and alloys thereof.

5. A method of rapidly and efliciently desulphurizing nickel particlesformed into strip of substantially 100% theoretical density and lessthan 0.25 inch in thickness and containing at least 0.02 weight percentsulphur as a contaminant which comprises heating said strip in areaction zone at a temperature below the melting point of nickel butabove 1500 F flowing hydrogen gas containing less than about 0.15hydrogen sulphide by volume through said reaction zone at a velocitygreater than about 400 centimetres per minute, and continuing saidheating 0 in said flowing hydrogen for from about 15 to about minutes tolower the sulphur content thereof to below about 0.005% by weight.

References Cited by the Examiner UNITED STATES PATENTS 2,159,231 I5/1939 Schlecht et a1. 29-4205 2,159,604 5/1939 Schlecht et al. 224

FOREIGN PATENTS 519,183 3/1940 Great Britain.

LEON D-. ROSDOL, Primary Examiner.

vCARL D. QUARFORTH, REUBEN EPSTEIN,

Examiners.

R. L. GOLDBERG, R. L. GRUDZIECKI,

Assistant Examiners.

1. A METHOD OF RAPIDLY AND EFFICIENTLY DESULPHURIZING A MATERIALSELECTED FROM THE GROUP CONSISTING OF NON-FERROUS METALLIC PARTICLES OFA SIZE SMALLER THAN ABOUT 300 MICRONS AND COMPACTED PRODUCTS FORMEDTHEREFROM AND WHICH CONTAIN AT LEAST 0.02 WEIGHT PERCENT SULPHUR AS ACONTAMINANT WHICH COMPRISES HEATING A LAYER OF THE SULPHUR CONTAMINATEDMATERIAL IN A REACTION ZONE AT A TEMPERATURE BELOW THE MELTING POINT OFSAID MATERIAL BUT ABOVE 1000*F.; FLOWING HYDROGEN GAS CONTAINING LESSTHAN ABOUT 0.15% HYDROGEN SULPHIDE BY VOLUME THROUGH SAID REACTION ZONEAT A VELOCITY GREATER THAN ABOUT 400 CENTIMETRES PER MINUTE; CONTINUINGSAID HEATING IN SAID FLOWING HYDROGEN FOR A PERIOD OF TIME UP TO ABOUT 2HOURS TO LOWER THE SULPHUR CONTENT OF SAID MATERIAL TO SUBSTANTIALLYLESS THAN ABOUT 0.02 WEIGHT PERCENT.